The invention is in the field of electronic reproduction technology and is directed to a method for calibration of an optoelectronic scanner element of a scanner device for point-by-point and line-by-line scanning of image originals. The a scanner device, also referred to as scanner, can be a black-and-white scanner for scanning black-and-white image originals, or can be a color scanner for scanning chromatic image originals.
Given a black-and-white scanner, a black-white image original is illuminated pixel-by-pixel by a light source and the scan light modulated by the brightnesses of the scanned pixels is converted with an optoelectronic converter into an image signal that represents the brightness values of the scanned image original between xe2x80x9cblackxe2x80x9d and xe2x80x9cwhitexe2x80x9d.
Given a color scanner, the scan light coming from the image original is first resolved with dichroitic filters into red, green and blue parts and is supplied to the three color channels of a color scanner. The chromatic light parts are then converted with optoelectronic transducers into three color signals that represent the color parts xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d of the pixels scanned in the color original.
The image signals or, color signals are further-processed in signal editing stages. The signal editing stages have a defined signal input range of which one corner value is referred to as a white level.
The total range of an image original to be scanned is matched to the defined signal input range of the signal editing stages by a calibration of a black-and-white scanner or color scanner before the beginning of scanning, in that the scan light coming from the brightest location of the image original, the white point, is converted into an image signal value or, into a color signal value per color channel that corresponds to the white level.
DE-A-25 45 961 has already disclosed a method for the automatic calibration of scanners. In a calibration phase, the color scanner element of a black-and-white scanner is positioned to the respective white point of the image original, and the scan light coming from the targeted white point is converted in the optoelectronic transducer into an actual image signal value. The actual image signal value is compared in a control unit to a rated image signal value that corresponds to the white level. A control signal modifies the gain of the optoelectronic transducer and/or of a following amplifier until the repetitive error is zero. The control signal value required for this purpose is stored for the duration of the original""s scanning that follows the calibration phase. The control unit is expanded to the three color channels for the white balance given color scanners.
The known method has the disadvantage that a corresponding white point on the image original to be reproduced must always be approached with the color scanner element in the calibration, this time-consuming and, particularly given repetitions of the a white balancing, being imprecise. Added thereto is that a brightest image location suitable as white point is often not present in a chromatic image original.
EP-A-0 281 659 recites a further method for the calibration of scanners, whereby the repeated approach of a white point with the color scanner element on an image original to be reproduced is avoided. For that purpose, a light attenuation factor is determined by optoelectronic scanning of the white point in the initial white balancing. Given repetitions of the white balancing, the scan light coming from the white point is simulated by the light of the light source attenuated corresponding to the identified light attenuation factor without renewed scanning of the white point in the image original, whereby the light attenuation is undertaken with a controlled iris diaphragm.
The known method is complicated and is based on a color-neutral density simulation, which is not always established in practice, and can therefore occasionally lead to unsatisfactory results. Further, no density simulation in the scanning of opaque originals is possible given the known method since the iris diaphragm is required for the correct setting of the depth of field.
It is therefore an object of the present invention to improve a method for the calibration of an optoelectronic scanner element of a scanner device for point-by-point and line-by-line scanning of image originals such that a calibration that can be implemented simply and in a short time is enabled.
This object is achieved according to the invention by providing a method for calibration of an optoelectronic scanner element of the scanner device for point-by-point and line-by-line scanning of the image, originals. The image original is illuminated and the scan light modulated with densities of the scanned image original is converted into image values with a light/voltage transducer unit. The white level is prescribed. The calibration of the scanner element is undertaken by changing a gain of the light/voltage transducer unit such that the image value generated when scanning the brightest location of the image original, the white point, corresponds to the predetermined white level. The light/voltage transducer unit is charged with a calibration light. The transducer densities as a criterion for attenuation of the calibration light respectively simulated by the different gains are measured from the measured image values given different gains of the light/voltage transducer unit. The identified transducer densities are allocated to the corresponding gains as a transducer density table. Diaphragm densities as a criterion for the attenuation of the calibration light achieved with the respective scanned diaphragms are determined from the image values that were measured given different scan diaphragms in the calibration light. The identified diaphragm densities are allocated to the corresponding scan diaphragms as a diaphragm density table. A reference diaphragm on the corresponding reference diaphragm density are determined from the diaphragm density table. Calibration is implemented with the identified reference diaphragm by setting the gain of the light/voltage transducer unit such that the image value acquired with the calibration light attenuated by the reference diaphragm corresponds to the predetermined white level. The reference transducer density belonging to the gain that has been set is determined from the transducer density table. The scan diaphragm for scanning the image original and the corresponding diaphragm density are determined from the diaphragm density table. An overall density is calculated from the reference diaphragm density, the reference transducer density, the diaphragm density of the scan diaphragm, and from the density of the white point of the image original. A gain of the light/voltage transducer unit that is allocated in the transducer density table to that transducer density that corresponds to the calculated overall density is determined. The gain that has been determined is set at the light/voltage transducer unit for scanning the image original.
The calibration method of the invention is composed of device-specific and master-specific steps.
In the device-specific steps, the characteristic of the light/voltage transducer unit is first registered, and the densities of the scan diaphragms and gray filters are identified and stored in the form of value tables. Subsequently, a device-specific, automatic calibration occurs. The device-specific steps need only be advantageously implemented at great time intervals or given a replacement of component parts in the scanner devices.
In the master-specific steps, the required gain of the light/voltage transducer unit is automatically identified merely from the previously stored value tables and the respective white point density of the image master to be scanned and is set at the light/voltage transducer means.
The preparation time for the originals"" scanning is significantly shortened as a result of the calibration method and the operator is relieved of routine calibration jobs.
The invention is explained in greater detail below with reference to the example of a black-and-white scanner on the basis of the drawing figure.